Approximately 1 in 10 patients without prior prostate biopsy undergoing surgery for lower urinary tract symptoms harbor incidental prostate cancer, however, practice guidelines do not provide recommendations for its management. We aimed at describing the oncologic outcomes of patients with grade group (GG) 1 and 2 prostate cancer diagnosed at transurethral resection of the prostate (TURP). This was a nationwide population-based observational study of patients undergoing TURP in Denmark from 2006 to 2022 using the Danish Prostate Registry. We estimated the cumulative incidence of further biopsies and magnetic resonance imaging (MRI), curative treatment, endocrine treatment, and cause-specific mortality with competing risk analyses. Among 24,494 patients who underwent TURP, there were 1016 men with GG 1 and 381 with GG 2. The 5-year cumulative incidence of further MRIs or biopsies was 36% (95% CI 33%-39%) for GG 1, and 30% (95% CI 25%-34%) for GG 2. Fifteen-year prostate cancer mortality was 8.4% (95% CI 5.3%-11%) for GG 1, and 14% (7.5%-21%) for GG 2. A total of 270 men with GG 1 underwent a biopsy after the TURP, and 162 (60%) had no cancer, in this group, prostate cancer mortality after 15 years was 0.6% (95% CI 0%-1.8%). Men with post-TURP biopsy of GG ≥ 2 had a prostate cancer mortality of 30% (95% CI 9%-50%) 15 years post-TURP. The major limitation was the heterogeneous follow-up, which could lead to an overestimation of prostate cancer mortality compared to a more standardized follow-up. We observed high prostate cancer mortality after TURP with GG 1 or 2, likely due to unsampled high-grade cancer in the peripheral zone. Patients with incidental prostate cancer should be further investigated to rule out high-grade cancer. For patients with GG 1 on TURP, once a subsequent biopsy does not show cancer, follow-up should be lessened similar to that of patients with an initial non-malignant biopsy.